The phenological and morphological response of populations of Koeleria cristata (L.) Pers. from Colorado and related areas in a transplant garden at Ft. Collins indicated marked heritable differences. Most significantly, and contrary to what would be predicted on the basis of other such studies involving elevational gradients, the highest collections from Colorado (about 2,850 m) were latest in development. The moisture regime of the native habitats was a more important selection force than the length of the growing season. Plants from the northern plains were similar in phenology and morphology to plants from the plains—foothills region of eastern Colorado. Two, and possibly three, regional relationships are suggested for the Colorado populations.